Here are some stats (focused on iSpot’s home base, the UK):
On the 27th Feb 2019:
iNat England users added 1,407 observations.
iNat Europe users added 7,864 obs,
iNat Global users added 52,772 obs
iSpot UK&Ireland users added 75 obs
iSpot Global users added 83 obs
So I’m not sure what you mean by team up, what is more likely to happen is iSpot users will drift to iNat.
I was with with South African community of iSpot for 5years until their IT team destroyed the community by removing all notifications of changes (comments, new IDs etc), which resulted in the SA community moving to iNat. I assume they have now fixed their unforgivable (IMO) error of judgement, but the functionality of the iSpot site remains rudimentary compared to iNat. The only thing I miss about iSpot is the species interactions. Please keep species interactions high on your ‘features to add’ list, Team iNat!